A common method of molding composite articles, known as the resin transfer molding process, involves placing a structure of reinforcing fibers, for example glass, carbon or aramid, into a mold and then injecting a liquid resin into the mold so that it penetrates though the entire reinforcing structure. The resin then cures producing a resin/fiber composite with good mechanical properties and relatively low weight. The final mechanical properties depend on the type of reinforcing fiber and resin employed. A conventional arrangement for transferring resin from a resin injection machine into the fibers in a mold is shown in FIG. 1 of the accompanying drawings. The nozzle from the resin injection machine is clamped to the injection port of the mold and pressurized resin is fed from the injection machine, entering the reinforcing fibers at the injection port. On large moldings more than one port may be used but this presents problems as the resin cures in the feed system and means must be provided for removing the cured resin.
In recent years there has been a significant increase in the possible complexity of moldings made by the resin transfer molding process brought about by the introduction of preforming which allows the assembly of complex configurations of reinforcing fibers together with, for example, the use of foamed plastics material inserts. Cycle times are normally quite long and the degree of complexity limited as the fiber reinforcement restricts the flow and these are primary restriction to an otherwise versatile process.
Composite structures of fibers in a polymer matrix offer low weight, high strength solutions to many engineering problems and their use in application is rapidly growing, and aircraft industries. Composites, typically of glass or carbon fibers in a thermoset or thermoplastic polymer matrix, have tensile strengths comparable with steels, densities in the region of 20% to 25% of steel, and a low modulus. It is the low modulus that is one of the major problems facing designers. Very often large increases are needed to provide equivalent stiffness compared to steel and this demands complex, often three dimensional, structures.
Complex three dimensional structures are therefore what the designer must employ to meet the structural demands of the product. These however, are not technically easy to produce or are prohibitively expensive owing to the long cycle times required.
There is a need therefore for an improvement form of molding process applicable to composite articles which will reduce cycle times, and hence costs, in the production of complex structures. The present invention seeks to provide such an improved method.
The use of a gallery or galleries in a core, which is otherwise resin-impermeable, allows resin to be ducted to wherever needed according to the mold configuration by employing a single resin injection needle. It is preferred that the injection nozzle is inserted into the gallery or one of the galleries in the core. The layer will often be in the form of a `skin` surrounding the core, and in this case it is preferred that the nozzle is inserted through the skin of the article to be molded into the gallery. On completion of the injection cycle the nozzle is removed prior to the resin curing and the injection port is plugged. The method of the invention allows very complex structures to be produced and cycle times reduced.
The main function of the core normally is to space the skins apart and prevent resin from filling the void between the skins. Any material which withstands the the forces imposed when closing the mold and the injection pressure of resin will in general be adequate. Thus cores are employed in relatively thick moldings and are usually of a closed-cell foamed plastics material, e.g. foamed polyurethane or polyester, for convenience, lightness and cheapness. They may however be of any other desired material such as plaster, wood, wax, and the like. Core materials such as wax offer the possibility of removal after moldings, by melting out. Hollow polymer cores, which could be pressurized, offer the possibility of molding components such as fuel tanks into the composite article. However, where the article is relatively thin and no core would be needed for the above purposes, then the `core` and the gallery may be one and the same. That is, the `core` may be present simply to define one or more galleries in the fiber layer.
While the above discussion has referred to a `resin` it will be appreciated that the process of the invention is equally applicable to other forms of molding composites, for example metal matrix composites whose ceramic fibers are contained within a mold and molten metal is injected into the cavity containing the ceramic fibers. The term `resin` is to be construed accordingly.
Additionally or alternatively to employing galleries in the core galleries can be provided in the fibers which make up the skin portion. These can be physical galleries, for example tubes inserted into, or holes cut through the fibrous layer, or preferably may be incorporated into the actual weave of the fiber layer. For example a row or rows of weft or warp rovings can be omitted from the weave leaving a gap which will in effect provide a gallery. Alternatively individual or small numbers of warp or weft rovings may be significantly smaller than the principal rovings again leaving a void space which acts as a gallery inducting the resin. However, especially in the latter case, it is preferred to use such galleries in addition to galleries formed in the mold core. It is the properties of the fibers used as reinforcing for composite materials, that is their relative stiffness, which ensures that such voids or galleries retain their integrity under the conditions of molding. The degree of compacting and the fact that the fibers are trapped in the mold accentuate this property.
In order to produce composite products which can be considered as structural members, that is having a high strength to weight ratio and being capable of replacing steel components, the following criteria will normally have to be fulfilled. These are that the reinforcing fibers are elongated and preferably continuous; orientation of the fibers being possible; and fibre density being high, preferably in the region of 60% by weight of the molded portion of the composite, or more.
To achieve the above it is necessary to pack the fibers into the mold very tightly. This considerably restricts the flow of resin and, indeed, at useful levels of reinforcing fiber the voids between them are in fact so small that they may be considered as capillaries. This being the case the voids are subject to the laws of fluid flow that apply to capillaries. In simple terms this means that the surface tension forces become more significant than the pressure used to force the polymer through the fiber layer and hence these control the rate of fluid flow.
It can be shown that when the internal diameter of a tube becomes small the force that can be applied to a fluid within it becomes very small and tends to zero. This is because the force applied is a function of pressure times area and since area reduces with the square of the diameter, as the diameter decreases the area decreases even more rapidly. The viscous resistance to flow through a narrow tube is given as follows. EQU P applied+P surface tension=P viscous resistance.
If P applied is small, that is where the effective diameter is small, it can be ignored and then ##EQU1## where v=fluid velocity
T=surface tension PA1 .theta.=angle of contact PA1 d=diameter of tube PA1 l=length of tube PA1 .mu.=viscosity
Solving for v we get ##EQU2## In any given situation T, d, and .theta. are constant and therefore the velocity of flow of resin is inversely proportional to the viscosity and the length of the tube. There are limitations to what can be achieved in lowering the viscosity for a given material. In the case of polymers, heating reduces viscosity but only to a limited extent. Hence if v, the fluid velocity is to be kept as high as possible then l, the tube length, must be kept to a minimum. It is the provision of galleries in the method according to the invention which enables l to be kept to a minimum within the fibre layer.
A further consideration is as follows. When a resin injection nozzle comes into contact with the fibre reinforcement layer, in effect only the area of the capillaries within the fibrous layer that project into the area of the nozzle is available to transfer liquid into the fibres. Because of the nature of fibre laminates, in reality most of the fluid that enters the laminate would do so via the capillaries that are available around the circumference of the nozzle. There is therefore a circular line of penetration through which the liquid enters the fiber laminate and it is the length of this line that becomes more important than the nozzle area. In the method according to the invention employing galleries of the type described more fully hereinafter, the lines of penetration are effectively very long. This again increases the rate of fluid flow into the fiber structure as will be apparent from the above discussion. The `galleries` employed to duct resin throughout the composite to be molded should have sufficiently large dimensions so as to avoid the restrictions on flow rates associated with capillary flow conditions thus allowing the resin to be ducted throughout the composite in a rapid manner enabling complex three dimensional articles to be produced in very much reduced cycle times. Furthermore, it is not necessary to employ high pressures in the method of the invention. As is apparent from the above discussion even high pressures do not significantly reduce cycle times where capillary flow conditions exist. Moreover high pressures require massive molds and ancillary equipment, and therefore increase molding costs. Typically the process of the invention will employ pressures of no more than 6-bar and often pressures of less than 3-bar.